The Work Habits of the North American Beaver

Castor canadensis, commonly known as the North American beaver, is a species with an interesting history. Beginning in the late 1600s and continuing into the early 1900s, beavers were hunted by European fur traders who, disappointed by the absence of spices in Canada, made their fortunes from beaver pelts, used mostly for hats. Because of this exploitation, by the 19th century beavers were close to extinction. Had it not been for the new European fashion for silk hats, the species would not be here today. The beaver, which is the national symbol for Canada and the state animal for both Oregon and New York, is a resident today on my farm in Afton, Virginia. This newcomer has drastically changed the pond on which it lives.

I became interested in the nature of this species, and the dam and lodge it had built. I could see these structures but had never seen the actual beaver; I was only aware of its presence through its handiwork. This was a great mystery to me, because I did not know how these massive structures were built, or why their shape and size changed over time. It seemed to me that once the dam was built and the water level of the pond was raised, there would be no need for the beaver to further change the pond's dynamics. I tested a null hypothesis: that the beaver builds his dam increasingly higher to achieve even higher water levels in the pond.

To execute my experiment, I first took a wooden post that was 3.4 cm by 1.8 cm by 180 cm. On this post I measured out and marked each centimeter and labeled it. This post would be used to measure the water height of the pond. Because the beaver might chew this down, I rubbed the pole with a substance called "4 the Birds," which is a transparent animal repellent. I hoped this would discourage the beaver from incorporating the measuring stick into the dam. Then I placed this post into the floor of the pond, about three feet away from the dam.

Post used to measure the water height of the pond

Gauge used to measure the effect rain had on the pond's water level

Figure 1: Water Level and Rain Over 39 Days(Click to enlarge)

Also, I used a 140-millimeter rain gauge to measure the natural effect that rain had on the water level of the pond. Since I wanted to study dynamic changes in the dam, I broke away a section of it and measured the sound of the water running out. For this part of my experiment, I used a decibel meter with a microphone. I used decibels in order to show how much the dam had been breached compared to the normal stillness of the water behind the dam. I held the device near the running water and measured the decibels that my breach created (See Figure 1.)

Water Height of the Pond Over 39 Days (Click to enlarge)

Figure 4: Diagram of Pond

For several days, I took measurements under normal circumstances, without any interference. I used this to establish my control. The water level of the pond stayed relatively consistent, at a height of 70 centimeters (See Figure 2). After four days of these measurements, I broke away a section of the dam that exited into a dry streambed (See Figure 4). Using a pick, I pulled the leaves, mud, and sticks of the dam away until the pond edge was visible. The resulting rush of water measured an average of 76 dB and traveled quickly down the streambed. The next day the dam had been repaired with more sticks and mud, and the water had ceased to flow.

Water flowing down the streambed after one of the three breaches of the dam.

A can on top of the dam before the rainfall.

I took measurements for seven more days, and then I breached the dam for a second time. I broke the dam in the same place, but this time I only removed about half the height of the structure. The water traveled down the streambed at a slow but steady rush of 56 dB. Once again, by the next day the beaver had repaired its dam, but there was a soda can stuck on top of it, implanted in the mud. Between that night and the next day, there was a substantial amount of rainfall, 103 mm. When I went out to take data, the water level of the pond had risen about 16 cm. The beaver had not built the dam high enough, and water was cascading over the dam at 67 dB. The next day there was 119 mm of rainfall. The water level of the pond had risen 9 cm, and water was once again crashing over the barrier. I thought the beaver had not changed his dam at all since the rain started, but to my surprise the can that had once been on top of the dam was now halfway under more sticks and mud. This indicated that the beaver had built its dam higher to hold back the rising water level of the pond, but that its efforts were insufficient.

The can further down under the new addition to the dam.

The can further down under the new addition to the dam.

The next day the rainfall was not nearly as much, only 15 mm. When I reached the pond, the beaver's efforts to hold up the dam were very evident. The soda can was now three-quarters of the way below the top of the dam, and only a small trickle was escaping the pond. After observing the pond with this dam height for some time, I again breached the barrier, this time pulling away the mud underneath the dam, which resulted in a water flow of 76.75 dB. The beaver did build it up again, but there was still a small amount of water escaping the dam, at 51 dB. This small trickle from the dam, with an average flow of 53 dB, was not repaired (See Figure 1). After six more days of taking data, I ended my experiment; the beaver did not build its dam higher than 70 cm.

My experiment rejected my null hypothesis: beavers do not keep building their dams ever higher. Rather, I established that beavers are content with a certain water level (in this case, 70 cm), and only change the height of the dam if nature's intervention changes the water level (rain). This is shown on days 15 and 16 (See Figure 1), when rain caused the water level to rise and the beaver built his dam higher in response. My results are consistent with the notion that the beaver's work levels are determined by external events, including rain, and the sound of rushing water caused by the rain.

The Beaver's Lodge

If I had time to continue this experiment, I would take a closer look into the way a beaver dam is built, and which components of the dam make it so strong and durable. Through this experiment, I have found that the dam is the crucial part of the beaver habitat, and since I now know what causes the beaver to build its dam, it is natural to inquire about the actual dam itself, which would give me more insight into the daily life and activity of the beaver.

The North American beaver is a fascinating species. At no point during my experiment, or since, have I actually seen the beaver that resides on our pond, but I have indirectly observed its building work. Though beaver fur was once treasured in Europe, beavers today are seen as pests because of their expanding population—they have recovered quickly in the past 200 years. The beaver is known as a "cornerstone species" that changes its habitat during its lifetime. I cannot be certain that our beaver visitor will stay forever, but the largest rodent in North America and Canada is for now is a concealed resident of my backyard.

References

Freedman, Bill. "Beavers". Gale Encyclopedia of Science. 2003.

Miller, James E., and Greg K. Yarrow. "Beavers". Prevention and Control of Wildlife Damage. (1994): B2-B10.